This application claims priority to International Application No. PCT/DE00/01564, which was published in the German language on May 17, 2000, and claims priority to German Application No. 19924347.6, filed May 27, 1999, and German Application No. 19936075.8, filed Jul. 30, 1999.
The present invention relates to a method and system for generating RZ signals.
A group of optical transmission systems which operate with high data rates requires sequences of short pulses which are denoted as RZ (return to zero) pulses. An optical pulse can in this case represent a logic 1, while a missing pulse corresponds to a logic 0 of a binary signal.
Patent DE 44 41 180 C1 discloses generating pulses by means of a ladder network of two modulators driven by sinusoidal voltages, which are operated at different frequencies. A frequency divider required to generate the second drive signal is disadvantageous and requires both modulators to be driven with high powers.
Generating short pulses has been disclosed in U.S. Pat. No. 4,505,587, in which ultrashort pulses are generated with the aid of a multiplicity of modulators, the frequencies of the drive signals being doubled in each case.
The generation of short optical pulses with the aid of phase-shifted voltages on the same frequency is known from Electronic Letters Aug. 5, 1993, Vol. 29, No. 16, pages 1449 to 1451. This phase shift must be exactly if the peak pulses are to have a constant shape.
In one embodiment of the invention, there is a method for generating RZ signals comprising feeding an optimal signal to a first modulator which has at least a first operating point for a first drive signal, wherein a transmission has a maximum, and the transmission has at least a minimum at peak values of a first drive signal; and feeding the optimal signal to a second modulator which has at least a second operating point which is spaced from a third operating point associated with the minimum transmission by xc2xc of the spacing between the operating points associated with a maximal and neighboring minimal transmission (1, 0) and the second modulator is fed a second drive signal having a amplitude is selected such that its peak values lie at least approximately for driving purposes between a minimum value of the transmission and half of the maximum value of the transmission.
In one aspect of the invention, the modulation is performed in Mach-Zehnder modulators.
In another aspect of the invention, in order to shorten the RZ pulses at least one additional modulation of the optical signal is carried out using at least one additional drive signal, the additional drive signal being at twice the frequency of the preceding drive signal.
In another aspect of the invention, the optical signal is modulated in binary fashion and feed to the series circuit of the first and second modulators.
In another aspect of the invention, a spatially periodic change to the sign of the modulated optical signal is preformed in one of the first or second modulators, a period length multiplied by an absolute value (|1/Velxe2x88x921/Vopt|) of the difference (1/Velxe2x88x921/Vopt) between the inverse (1/Vel) of a propagation rate (Vel) of the first and second drive signals and an inverse (1/Vopt) of a propagation rate (Vopt) of the optical signal yields at least approximately an inverse (1/Fel) of an electric modulation frequency (Fel).
In another embodiment of the invention, there is a system for generating RZ signals with a plurality of series-connected modulators which are fed an optical signal comprising a series circuit having at least two modulators, the modulators are fed signals having the same frequency but different effective amplitudes with a ratio of 1:4, wherein the at least two modulators have different operating points, a transmission having a maximum at one operating point, and the other operating point being selected such that the transmission has a minimum value only at a peak value of the second drive signal.
In another embodiment of the invention, there is a system for generating RZ signals comprising at least two series-connected modulators are integrated on a chip which are fed an optical signal, wherein a electric drive, signal is routed through the modulators; and a matching element located between the modulators to provide amplitude and/or phase matching.